Velocity-shear-induced ion-cyclotron turbulence: Laboratory identification and space applications

Abstract

Laboratory measurements are reported that identify a new class of plasma oscillation driven by the inhomogeneity in wave energy density caused by transverse-velocity shear [Ganguli et al., Phys. Fluids 31, 823 (1988)]. The experiments concentrate on a branch of oscillation in the ion-cyclotron range of frequencies that results from the coupling of the magnetic-field-aligned current and the inhomogeneous dc electric field localized in a layer thicker than the ion gyroradius. The observed transition between the well-known current-driven electrostatic ion-cyclotron mode and this inhomogeneous energy–density-driven mode is related to the ratio of the azimuthal and axial Doppler shifts. The mode characteristics associated with the two instabilities have significantly different properties. For conditions of large transverse-velocity shear, turbulence is generated with a broadband, spiky spectrum around the ion-cyclotron frequency at small values of the magnetic-field-aligned current. The experimental identification is reinforced with numerical results from a nonlocal eigenvalue condition.